The dimensions of the RF shield and the RF frequency for proton imaging at 7 T allow the excitation of a propagating TE11 waveguide mode, which has given rise to the concept of traveling wave imaging (see, e.g., Brunner et al., Nature, 457, 994-999 (2009)). RF excitation is typically achieved by using a patch antenna or crossed dipole antenna placed at one end of the bore. A nearly uniform B1+ field is produced along the entire length of the bore when it is empty. With a human body in the bore, dielectric boundaries and tissue conductivity can give rise to standing wave behavior and attenuation of the B1+ field, resulting in a highly non-uniform excitation. The strongest B1+ field and maximum SAR occur in the parts of the body closest to the antenna such as the head and shoulders or legs, with very little excitation in the torso. To create a useful excitation in the torso, it may be necessary to deposit large amounts of energy in the head or legs. Some methods have been described for creating structures which guide the traveling wave to deliver the power more efficiently to more distant regions. It would be desirable though to excite the TE11 traveling wave mode at or near the mid section of the bore, close to the imaging volume of the scanner.
Thus, it may be beneficial to address and/or overcome at least some of the deficiencies described herein above.